Field of the Invention (Technical Field)
The invention relates to a method of degrading halogenated organic compounds, to a use of said method for decontaminating PFOS (perfluorooctanesulfonic acid) and PFOA (perfluorooctanoic acid) contaminated medium and to a composition such as that which is suitable to be applied in said method.
Description of Related Art
Halogenated organic compounds are equivalents of organic compounds (i.e. compounds comprising at least carbon and hydrogen) wherein some, most, or all, of carbon-linked hydrogen atoms that would have been present in an equivalent organic compound are instead halogen atoms. The halogen atom(s) may be chlorine, fluorine, bromine, iodine, or a combination thereof. Perhalogenated organic compounds are a special class of halogenated organic compounds wherein all of the carbon-linked hydrogen atoms that would have been present in an equivalent organic compound are halogen atoms. As an example, perfluorinated organic compounds (PFCs) are perhalogenated organic compounds with fluorine in place of hydrogen. It is noted that one or more further functional groups attached to a carbon may be present as well.
A class of PFCs that has been widely used in the production of commercial or industrial products, and released into the environment, are perfluorinated fatty acids (PFFAs). PFFAs are synthetic, fully fluorinated analogues of fatty acids characterized by a perfluoro-alkyl chain and a terminal sulfonate or carboxylate head group; chain lengths are typically in the range of 4-28 carbon atoms. Well-known examples of PFFAs are PFOS (perfluorooctanesulfonic acid) and PFOA (perfluorooctanoic acid): both are surfactants; amongst other applications, they have been used in fire-fighting foams, non-stick polymers, coatings for paper and textiles, oxidative protective coatings on metals, as inert surfactants for semi-conductor etching and as thermally stable lubricants. Further examples are perfluoroheptane sulfonic acid (PFHpS (C7S)), perfluorohexane sulfonic acid (PFHxS (C6S)), perfluoropentane sulfonic acid (PFPeS (C5S)), perfluorobutane sulfonic acid (PFBS (C4S)), perfluoroheptanoic acid (PFHpA (C7A)), perfluorohexanoic acid (PFHxA (C6A)), perfluoropentanoic acid (PFPeA (C5A)) and perfluorobutanoic acid (PFBA (C4A)). Cn (where n is an integer) relate to the length of the carbon chain; S identifies the compound as a sulfonic acid, A identifies the compound as a carboxylic acid.
Perfluorinated organic compounds such as PFFAs are very stable with respect to photolysis, thermolysis, chemical degradation (e.g. oxidative degradation), microbial degradation, and metabolism by animals. Whilst this is advantageous for many applications, it poses serious problems with respect to their disposal. Many PFCs, such as PFFAs are environmentally persistent. Due to their stability with respect to degradation and metabolism, it was long thought that PFFAs were non-toxic (Sargent and Seffl 1994). More recent evidence has revealed that this is not the case and shown that at least certain PFFAs are biologically active and can cause peroxisomal proliferation, increased activity of lipid and xenobiotic metabolising enzymes, and alterations in other biochemical processes in exposed organisms (Obourn et al 1997; Sohlenius et al. 1994). Bioaccumulation and biomagnification of PFFAs is problematic. PFOS and PFOA have recently been designated as persistent organic pollutants and have been added to Annex B of the Stockholm convention (Moermond et al., 2010). PFOS is an environmentally persistent degradation product of many perfluorinated organic compounds. Therefore there is a need for a reliable and effective method to degrade said compounds. For PFOS degradation using oxidation or reduction techniques, it is very likely that intermediate compounds will occur. Most likely smaller fluorinated sulfonates and acids will be created.
Many methods for degrading certain classes of halogenated organic compounds are known in the prior art. The majority of these are not suitable for degrading fluorinated and in particular perfluorinated organic compounds such as PFFAs, nor are they suitable for use in environmental “cleanup” applications, i.e., for removing the halogenated organic compounds from the environment, such as in soil remediation, e.g., because no or almost no degradation is obtained. Even if some degradation is obtained e.g. large quantities of degrading chemicals are required or elevated temperatures must be applied.
Fenton's reaction is one example of a reaction that previously has been applied for degrading halogenated organic compounds. The reaction has been applied, for example, for degrading chlorinated environmental contaminants e.g. trichloroethylene (TCE), tetrachloroethylene (PCE), perchloroethylene, perchlorinated biphenyls, etc.
An alternative reaction that has been used to degrade halogenated organic compounds is to use a persulfate in the presence of a metal catalyst. However typically relatively large quantities of catalyst and persulfate have been used and typically at elevated temperature.
In 2008 ARCADIS UK supported research at Imperial College London into oxidative and reductive treatment methods, including those mentioned above, for degrading PFOS in soil and/or ground water. Whilst certain oxidative methods achieved up to almost 100% degradation of the PFOS, these methods involved introducing high concentrations of oxidants and elevated temperatures. The conditions applied in these tests that resulted in effective degradation cannot reasonably, safely or cost effectively be applied in practice to reduce or preferably eliminate PFOS in the environment such as in contaminated soil and/or ground water.
Further tests were executed at ARCADIS NL to determine whether PFOS could be degraded using the set-ups of the experiments performed at ARCADIS UK at more practical, i.e. lower concentrations. The tested methods were methods that are known to be effective for degrading other halogenated organic compounds such as perchlorinated organic compounds and included using temperature activated persulfate, Fenton's reaction, a reaction using hydrogen peroxide and ferrous iron activated persulfate (i.e. using chelated iron) and catalysed persulfate oxidation. Effective degradation was only achieved at e.g. concentrations which are too high (such as from 20% w/w to 80% w/w active compound or combination of active compounds) for practical application in the field. Tests were particularly disappointing in terms of e.g. effectiveness wherein soil and water containing PFOS were combined.
To the inventors knowledge, of the methods of the prior art, only permanganate is known to be somewhat effective for degrading perfluorinated organic compounds such as PFOS (Liu et al. 2011). Disadvantages of using permanganate for degrading halogenated organic compounds, in particular for environmental “cleanup” applications such as in soil remediation, are that elevated temperatures are required and that insoluble manganese dioxide precipitates are formed. Precipitation of manganese dioxide may cause clogging of the soil, resulting in reduced permeability of the soil to water. As a further result, this can prevent proper decontamination of the site by inhibiting oxidant access. Example conditions of the prior art for degrading PFOS with permanganate in buffered aqueous solution are a temperature of 65° C. and a pH of 4.2; under these conditions, identified as being particularly effective by Liu et al., 46.8% degradation was achieved after maintaining these conditions for 18 days, which is somewhat disappointing.